What the word actually means
Samādhi is from the Sanskrit sam-ā-dhā, to put together. The image is of bringing what was scattered into unified placement. In the Yoga Sūtras, samādhi is the state in which the mind has stilled the modifications (citta-vṛtti) that ordinarily occlude direct perception, and what remains is the clear awareness in which the object known and the awareness knowing it are no longer experienced as two.
The grades
Patañjali distinguishes savikalpa samādhi (still some subtle subject-object structure remaining) from nirvikalpa samādhi (that structure dissolved). Later Vedānta added sahaja samādhi — the natural, continuous form in which the absorption is no longer a special state but the practitioner's ordinary mode. Ramana Maharshi taught that any temporary samādhi attained on the cushion that does not become sahaja in daily life is incomplete: a flash, not a recognition.
Distinction from absorption in concentration
Samādhi is sometimes confused in popular English with the deep concentration states of jhāna (Pāli; dhyāna in Sanskrit). They are related — dhyāna is the seventh limb, immediately preceding samādhi — but distinct. Jhāna/dhyāna is a sustained absorption in an object; samādhi is the further dissolution of the subject-object distinction itself. Confusing the two has produced a lot of unnecessary disagreement between Theravāda Buddhist and Vedāntic teachers about what the goal of meditation actually is.
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